Sunflower (Helianthus annuus L.) growth and yield are severely compromised by boron (B) deficiency in alkaline calcareous soils of Pakistan. Biochar produced through low-temperature pyrolysis helps to maintain acidic functional groups in such soils. The aim of this study was to enhance B availability in biochar-amended alkaline calcareous soils in order to improve morpho-physiological and yield attributes of sunflower. In a pot experiment, sunflower was grown with three fertiliser treatments: Diammonium phosphate (DAP) alone; simple B + DAP; and DAP coated with biochar and B (BC-BDAP). All treatments were grown in soil receiving no biochar, and the BC-BDAP treatment was also applied in soil amended with wheat straw biochar at 2% and 4%. Preparation with wheat straw biochar improved soil properties such that soil with 4% biochar showed highest levels of organic matter (0.97%), total nitrogen (0.08%), available phosphorus (6.21 mg/kg), extractable potassium (93.4 mg/kg) and available B (0.41 mg/kg). Furthermore, compared with DAP alone and B + DAP, BC-BDAP in 4% biochar-amended soil enhanced the concentrations of shoot B (by 64% and 35%) and root B (by 41% and 26%), respectively. This indicated sufficient B availability in soil for root absorption, which subsequently increased sunflower achene weight per plant (by 33% and 17%), protein content (by 29% and 18%) and oil content (by 17% and 10%). Thus, application of BC-BDAP fertiliser in 4% biochar-amended soil can be an efficient strategy for enhancing B availability in alkaline calcareous soils and increasing sunflower growth and yield.